When starting the Mendix Runtime you can choose the menu option synchronize modeler file. If the metamodel and the database are not in sync you get the question if you want to sync it automatically or manually (the queries can be written to a file and executed by hand on the database).
Changes in the metamodel are automatically propagated to the database while retaining the data as much as possible. However, not for every change the data can be retained.
Yes, you can! Changes in the metamodel are propagated to the data database and data is retained. The runtime creates SQL queries to update the database to reflect the metamodel again. Of course, I would recommend always checking your application in a test environment first.